(1) Field of the Invention
The present invention relates generally to rotatable power transmitting devices, and more particularly, to a hub structure for adapting such rotative devices to shafts of different diameters.
(2) Description of the Prior Art
Stock sheaves are manufactured in certain standard size both with respect to different sheave diameters for a given shaft diameter and different shaft diameters for a sheave of given diameter. A well-known structure for mounting sheaves, sprockets, couplings, gears and similar machine elements onto shafts consist of a tapered bushing having a longitudinal slit extending the length thereof and a cylindrical bore therethrough for gripping the shaft internally and seating on a corresponding tapered surface in the bore of the hub of the machine element. One such bushing which has been particularly successful is set out in U.S. Pat. Nos. 2,452,458 and 2,487,128, issued to Hahn.
The bushing as taught by Hahn consists of a tapered body of the frustroconical shape having a radial annular flange disposed on the large end of the tapered body. The flange usually contains three plain unthreaded holes for receiving screws to extend through the flange into threaded bores at the end of the hub. Screws are inserted through the flange into the corresponding threaded holes in the hub and, as the screws are tightened, the tapered surface of the bushing and hub are drawn together. This, in turn, causes the slit bushing to contract about and securely grip the shaft, thereby securing the assembly together and preventing relative rotation between the shaft, bushing, and hub.
Reverse mounting is also possible where, for example, there is limited space adjacent the flange end of the bushing. In this case, screws are inserted through three plain, unthreaded holes in the hub and into corresponding threaded bores in the flange. As the screws are tightened, the tapered surfaces are drawn together and the bushing contracts around the shaft as described above. A key with a corresponding keyway may be provided between the shaft and the bushing to further guard against relative rotation between the members.
The bushings are normally produced either by machining the bushings from blanks of cast iron or by a powdered metal process which eliminates much of the machining.
Certain disadvantages with the above design become apparent. First, improper alignment of the bores and the flange with the corresponding bores in the hub greatly increase the difficulty in assembly. In fact, some disassembly of the machine itself may be necessary before the hub and flange of the bushing can be properly aligned. Second, bushings are subjected to very high torque especially where frequent reversal rotation occurs. In addition, stresses are concentrated in certain areas of the bushings such as the junction between the flange and the barrel. While a useful and inexpensive material, cast iron is generally not malleable and is subject to fracture. As a result, the prior art cast iron bushings are frequently broken when attempts are made to remove the adapter hub from the hub and shaft assembly.
Several approaches have been taken to overcome the problem of alignment of the flange and the hub. U.S. Pat. Nos. 2,441,467, issued to Browning, and 2,556,151, issued to Bremer, both disclose a detachable bushing for joining a wheel to a shaft. These patents teach a two-piece bushing having an outer flange and inner split tapered bore. One end of the tapered bore adjacent to the flange includes a circumferential groove adapted for receiving a bolt having a capped head for mating within the groove. These bolts, once secured to the pulley or the like, serve to draw the bushing into tight gripping arrangement with the shaft when they are threaded in one direction and serve to force the bushing out of gripping or frictional engagement with the shaft when threaded in the opposite direction.
A similar function is illustrated in U.S. Pat. No. 2,447,299, issued to Williams, in which a screw collar is combined within a circumferential groove located at one end of a tapered bore. The screw collar is adapted to engage an internal thread in the wheel bushing bore so when it is turned in one direction the sleeve will be shifted into binding engagement with the shaft surface in the bushing bore and, when turned reversibly, will release the sleeve. The screw collar is split longitudinally to reduce the thickness at a point opposite the split in order that the screw sleeve can be opened up for placement onto the groove and afterwards compressed for permanent retention.
Certain disadvantages become apparent with such designs. First, the bushing assemblies as taught by Bremer and Browning both localize the application of force on the bushing by means of the screw heads along three or at most four locations. In addition, because part of the screw cap forms the abutment, a bending moment is applied to the screw body which may result in premature failure. Finally, the rotation of the cap with respect to the bushing abutment surface makes accurate measurements of torque difficult or impossible and may damage the surfaces of the bushing in the caps.
Likewise, the fastening means taught by Williams, makes accurate measurement of the torque applied impossible. In addition, Williams requires a special bushing to receive the screw collar. Finally, because the screw collar is split, this presents a safety hazard when used in high-speed rotating machinery since the collar may become separated or broken during operation.
It has thus become desirable to develop a bushing for securing pulleys, sprockets, couplings and the like to rotatable shafts which is easy to align and remove while, at the same time, eliminating the prior art problems of localized stresses or limited use to specific hubs, and which is inherently safe for use with high-speed rotating equipment.